The present invention relates generally to fluid-flood secondary recovery operations for extracting oil and gas from subsurface rock formations, and more particularly to a method of increasing the efficiency of such operations by increasing permeability and porosity of the rock.
Secondary hydrocarbon recovery, and it should be understood that this term is meant to include what is sometimes referred to as "tertiary" recovery, is of extreme importance. Without it, only a small fraction of all hydrocarbons proven to be present in subsurface formations would be recoverable. Secondary recovery can increase the amount recovered to between 40 and 50% of the oil-in-place.
In water and other fluid-flood methods of secondary recovery, a fluid is injected under pressure into a reservoir through an injection well. This fluid displaces the hydrocarbons, forcing them to migrate toward producing wells. Formation brine is one of the commonly used sources for the displacing fluid. Other sources include fresh water from ponds, lakes, streams, rivers, and subsurface formations, and salt water from subsurface formations and the ocean. The choice is governed by such factors as availability and compatibility of the water with the formation to be flooded.
The injection rate and pressure to be used in a given fluid-flood operation are selected to provide an economic operation while avoiding excessive high pressure that might induce detrimental fractures in the formation. The efficiency of a flood project is dependent upon many factors, such as the relative permeability of the producing formation to oil, gas, and displacing fluid, the porosity of the formation, the viscosities of the various fluids, and the irreducible residual hydrocarbon saturation.
The particular grains of a rock are usually held together by a cementing material such as calcite, dolomite, quartz, iron oxide, anhydrite, and various others. The porosity, i.e.--the voids within which the hydrocarbons are found, and the permeability, i.e.--the extent to which such voids are interconnected thus permitting the hydrocarbons to migrate, as, for example, toward a producing well, are largely determined by the grain size, the degree of compaction of the mineral grains, and the extent of their cementation.
A vast majority of the oil and gas bearing rock strata are of sedimentary character, that is they comprise sandstones, shales, or limestones, or combinations thereof. In turn, the most important building blocks of such rock types are the silicate and carbonate minerals. Oxides, sulfides, and sulfates are found in the rock matrices less frequently. Among the cations most commonly associated with these compounds are the metals of Groups I and II, such as, for example, aluminum and iron.